The Vertical Cavity Surface Emitting Laser diode, hereinafter referred to as a VCSEL, is attractive as a device which may be produced by planar technology and as a class of devices with a wide range of potential uses including optical communications, optical discs, laser printers and light sensing systems. In the VCSEL, the lasing cavity is perpendicular to the optical surface of a laser chip. Therefore, high packing density, compared to the packing density of edge-emitting lasers with the lasing cavity parallel to the surface of the laser chip, is obtainable. This leads to a promising future in high density laser arrays, high data transmission in optical communication systems, high parallel processing in optical communication systems, as well as supplying a route for fast and high capacity data transmission between electronic chips. Furthermore, the radial symmetry of their beams makes them suited for beam-combining with cylindrical fibers.
In the VCSEL the light output is in the film growth direction which is usually parallel to the direction of the injection current. Due to this feature, the mirror through which laser emission takes place and the electrical contact physically occupy the same side of the laser structure, i.e. either on the top or on the bottom of the device. Typically, the mirror is located approximately in the center of the surface while the electrode is located peripherally of the mirror. For example, see Kenichi Iga, "Recent Advances of Surface Emitting Semiconductor Lasers," Optoelectronics-Devices and Technologies, Vol. 3, No. 2, December 1988, pp. 131-142, and L. M. Zinkiewicz et al., "High Power Vertical-Cavity Surface-Emitting AlGaAs/GaAs Diode Lasers," Appl. Phys. Letters, Vol. 54, No. 20, May 15, 1989, pp. 1959-1961.
An attempt to simplify the construction of a VCSEL by combining the mirror and the electrode into a single unit led to relatively low quantum efficiencies. See Deppe D. G. et al., "AlGaAs-GaAs and AlGaAs-GaAs-InGaAs vertical cavity surface emitting lasers with Ag mirrors," Journal of Applied Physics, Vol. 66, No. 11, Dec. 11, 1989, pp. 5629-5631. The mirrors comprised a 0.55 .mu.m thick reflective Ag mirror which also acted as the electrode of the laser. The emission took place through the .lambda./4 reflector semiconductor stack arranged opposite to the mirror/electrode. U.S. Pat. No. 4,949,351 issued Aug. 14, 1990 to Koich Imanaka discloses a Ti, Pt and Au layered structure with a total thickness of 900 .ANG., which is used as an electrode-mirror. An article by E. F. Schubert et al., "Low-threshold vertical cavity surface-emitting lasers with metallic reflectors", Applied Physics Letters, 57 (2), Jul. 9, 1990, p. 117-119, L. W. Tu et al., "Vertical-Cavity Surface Emitting Lasers With Semi-Transparent Metallic Mirrors And High Quantum Efficiencies", Applied Physics Letters, 57(20), Nov. 12, 1990, pp. 2045-2047, and U.S. application Ser. No. 07/526,204, filed May 21, 1990 (Deppe D. G. disclose a VCSEL with a metallic mirror which simultaneously acts as an electrode of the device with a thickness sufficient to permit lasing emission through the mirror-electrode. However, while the quantum efficiency of the latter is improved over the structure of Deppe D. G. or Imanaka, there is still a substantial loss in transmission of the lasing emission through the metallic mirror-electrode.
Therefore, there is still a need for a VCSEL with an improved quantum efficiency and light transmission and which could be also produced in a simplified manner utilizing planar technology.